In presses and dies, it has been common to utilize die cylinders which communicate with a manifold and function as die springs. It has also been suggested that self contained gas springs be provided as shown in U.S. Pat. Nos. 4,742,997 and 4,741,518.
A major consideration in connection with the design of the self contained gas springs is that when charged with gas, such as nitrogen, at high pressures on the order of 1000 p.s.i., the normal stroke of the piston therein substantially increases the pressure. Accordingly, the construction of the self contained die cylinder must be constructed to withstand such high pressures. This results in the need to increase the geometric spaces in which the die cylinders are used. It has thus been customary to compromise the design.
Another consideration in connection with self contained gas springs is that in time gas may pass past seals. It has been heretofore suggested that one-way valves be provided to vent the gas from a space into which the gas has passed. Typical constructions that have been suggested comprise a spring loaded ball valve as shown in co-pending application Ser. No. 07/783,206, filed Oct. 28, 1991, now U.S. Pat. No. 5,172,892 incorporated herein by reference.
Such a construction is difficult to make and assemble as well as calibrate for desired venting.
Among the objectives of the present invention are to provide a self contained gas spring which is functional and operates at relatively high pressures; which incorporates a pressure relief construction for venting a space into which gas has passed within the cylinder that is dependable, reliable and can be more readily manufactured; which does not increase the pressure substantially during the movement of the piston within the cylinder; which is compact; and which requires a minimum of maintenance.
In accordance with the invention, a self contained gas spring, comprises a cylindrical housing having a closed end, an open end and a cylindrical bore. A cylindrical sleeve extends into the bore of the housing. The sleeve has at least a portion thereof forming an internal cylindrical surface. A piston rod extends through an opening in the sleeve and has a piston engaging the cylindrical surface. A portion of the sleeve defines a first space between the sleeve and the bore of the housing. The first space communicates with a second space between the piston end of the piston rod and the closed end of the housing. A charging valve is provided in the closed end of said housing for charging the space at the piston end of said piston rod and the space between the sleeve and the housing with gas under pressure. A third axial space is provided between the cylindrical surface on the sleeve and the piston rod above the piston. An annular groove is provided on the outer surface of the sleeve and circumferentially spaced passages extend from the groove to the first space. The groove has sides that converge inwardly toward the passages. An annular elastic seal sealingly engages the converging surfaces of the groove such that when the pressure in the third space exceeds the pressure in the first space, the annular seal is tensioned such that at least a portion thereof is flexed out of engagement with the diverging surfaces to vent the third space into the first space.
In a modified form, the annular groove and annular seal are provided on the piston rod and functions to vent the third space into said first space.
This invention is also applicable to die cylinders having an open end that communicates with a gas manifold.